The present disclosure relates to a lithium ion secondary battery negative electrode containing a negative electrode active material which can occlude and release lithium ions, a lithium ion secondary battery including the negative electrode, and a battery pack, an electric vehicle, an electricity storage system, a power tool, and an electronic apparatus, which include the secondary battery.
In recent years, electronic apparatuses typified by cellular phones, personal digital assistants (PDA), and the like have become widespread, and further miniaturization, weight reduction, and extension in life thereof have been desired intensely. Along with this, batteries, in particular secondary batteries, which are small and lightweight and which can obtain a high energy density, have been developed as power sources. Recently, application of these secondary batteries to not only the above-described electronic apparatuses, but also various uses typified by battery packs, electric vehicles e.g., electric cars, electricity storage systems, e.g., home electricity servers, and power tools, e.g., electric drills, has been studied.
Secondary batteries based on various charge and discharge principles have been proposed widely. Among them, a lithium ion secondary battery through the use of occlusion and release of lithium has a potential. This is because an energy density higher than those of a lead battery, a nickel cadmium battery, and the like can be obtained.
The lithium ion secondary battery is provided with an electrolytic solution in addition to a positive electrode and a negative electrode, and the negative electrode contains a negative electrode active material capable of occluding and releasing lithium ions. As for the negative electrode active material, carbon materials, e.g., graphite, have been used widely. Meanwhile, a further increase in battery capacity has been desired recently. Consequently, use of Si has been studied. The theoretical capacity of Si (4.199 mAh/g) is especially larger than the theoretical capacity of graphite (372 mAh/g) and, therefore, a significant increase in battery capacity can be expected.
However, if Si is used as the negative electrode active material, cracking occurs mainly in the vicinity of the surface layer of the negative electrode active material easily because of significant expansion and shrinkage of the negative electrode active material during charge and discharge. When the negative electrode active material is cracked, a highly-reactive fresh surface (active surface) is generated and, thereby, the surface area of the negative electrode active material (reaction area) increases. Consequently, a decomposition reaction of the electrolytic solution occurs on the fresh surface and, in addition, the electrolytic solution is consumed to form a coating film derived from the electrolytic solution on the fresh surface. Therefore, the battery characteristics, e.g., cycle characteristics, are degraded easily.
In order to improve the battery characteristics, e.g., cycle characteristics, various studies on the configuration of the lithium ion secondary battery have been made.
Concretely, in order to improve the cycle characteristics and the safety, Si and amorphous SiO2 are deposited at the same time by using a sputtering method (refer to Japanese Unexamined Patent Application Publication No. 2001-185127, for example). In order to obtain excellent battery capacity and performance of safety, an electrically conductive material layer (carbon material) is disposed on surfaces of SiOx particles (refer to Japanese Unexamined Patent Application Publication No. 2002-042806, for example). In order to improve high-rate charge and discharge characteristics and cycle characteristics, a negative electrode active material layer is disposed in such a way that Si and O are contained and the oxygen ratio increases in the side near to a negative electrode collector (refer to Japanese Unexamined Patent Application Publication No. 2006-164954, for example). In order to improve the cycle characteristics, a negative electrode active material layer is disposed in such a way that Si and O are contained, the average oxygen content as a whole becomes 40 atomic percent or less, and the average oxygen content increases in the side near to a negative electrode collector (refer to Japanese Unexamined Patent Application Publication No. 2006-114454, for example). In this case, the difference between the average oxygen content in the side near to the negative electrode collector and the average oxygen content in the side far from the negative electrode collector is specified to be 4 atomic percent to 30 atomic percent.
In order to improve initial charge and discharge characteristics and the like, a nanocomposite containing a Si phase, SiO2, and an MyO metal oxide is used (refer to Japanese Unexamined Patent Application Publication No. 2009-070825, for example). In order to improve cycle characteristics, powdered SiOx (0.8≦x≦1.5, particle diameter range of 1 μm to 50 μm) and a carbonaceous material are mixed and fired at 800° C. to 1,600° C. for 3 hours to 12 hours (refer to Japanese Unexamined Patent Application Publication No. 2008-282819, for example). In order to reduce an initial charge and discharge time, a negative electrode active material represented by LiaSiOx (0.5≦a-x≦1.1 and 0.2≦x≦1.2) is used (refer to International Publication No. 2007/010922, for example). In this case, Li is evaporated on an active material precursor containing Si and O. In order to improve charge and discharge cycle characteristics, the composition of SiOx is controlled in such a way that the molar ratio of the amount of O relative to the amount of Si in a negative electrode active material becomes 0.1 to 1.2 and the difference between a maximum value of the molar ratio of the amount of O relative to the amount of Si and a minimum value thereof in the vicinity of the interface between the negative electrode active material and a collector becomes 0.4 or less (refer to Japanese Unexamined Patent Application Publication No. 2008-251369, for example). In order to improve load characteristics, a Li-containing porous metal oxide (LixSiO, where 2.1≦x≦4) is used (refer to Japanese Unexamined Patent Application Publication No. 2008-177346, for example).
In order to improve charge and discharge cycle characteristics, a hydrophobized layer of a silane compound, a siloxane compound, or the like is disposed on a thin film containing Si (refer to Japanese Unexamined Patent Application Publication No. 2007-234255, for example). In order to improve cycle characteristics, an electrically conductive powder, in which the surface of SiOx (0.5≦x<1.6) is covered with a graphite coating film, is used (refer to Japanese Unexamined Patent Application Publication No. 2009-212074, for example). In this case, it is specified that broad peaks appear at 1,330 cm−1 and 1,580 cm−1 in the raman shift of a raman spectrum with respect to the graphite coating film and the intensity ratio I1330/I1580 thereof satisfies 1.5<I1330/I1580<3. In order to improve a battery capacity and cycle characteristics, a powder containing 1 percent by mass to 30 percent by mass of particles having a structure, in which Si microcrystals (size of crystal=1 nm to 500 nm) are dispersed in SiO2, is used (refer to Japanese Unexamined Patent Application Publication No. 2009-205950, for example). In this case, regarding the particle size distribution on the basis of a laser diffraction-scattering particle size distribution measuring method, the cumulative 90% diameter (D90) of the powder is specified to be 50 μm or less and the particle diameter is specified to be less than 2 μm. In order to improve cycle characteristics, SiOx (0.3≦x≦1.6) is used and, in addition, a pressure of 3 kgf/cm2 or more is applied to an electrode unit during charge and discharge (refer to Japanese Unexamined Patent Application Publication No. 2009-076373, for example). In order to improve overcharge characteristics, overdischarge characteristics, and the like, a Si oxide, in which the atomic ratio of Si to O is 1:y (0<y<2), is used (refer to Japanese Patent No. 2997741, for example).
In addition, in order to electrochemically accumulate or release large amounts of lithium ions, an amorphous metal oxide is disposed on the surfaces of primary particles of Si or the like (refer to Japanese Unexamined Patent Application Publication No. 2009-164104, for example). The Gibbs free energy in oxidation of the metal to form this metal oxide is smaller than the Gibbs free energy in oxidation of Si or the like. In order to realize a high capacity, a high efficiency, a high operating voltage, and a long life, use of a negative electrode material, in which the oxidation number of a Si atom satisfies a predetermined condition, has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2005-183264, for example). This negative electrode material contains Si with an oxidation number of 0, a Si compound having a Si atom with an oxidation number of +4, and a Si lower oxide with an oxidation number of more than 0 and less than +4.
In order to suppress an increase in impedance of a whole negative electrode, use of a composite negative electrode active material including Si-containing particles, carbon nanofibers attached to the surfaces of the Si-containing particles, and a catalyst element, e.g., Cu, to facilitate growth of the carbon nanofibers has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2007-165078, for example).